1. Field of The Invention
The present invention relates to a coil assembly for use in an evaporative heat exchange apparatus in which the coil assembly is to be mounted in a vertically oriented duct or conduit of a duct or conduit of the apparatus in which heat exchange fluids, typically a liquid, usually water, and a gas, usually air, flow externally through the coil assembly to cool or condense a heat transfer fluid passing internally through the tubes of the coil assembly. More particularly, the coil assembly of the present invention is most effectively mounted in a counterflow evaporative heat exchanger so that water flows downwardly and externally through the tube assembly while air travels upwardly and externally through the coil assembly.
The coil assembly of the present invention can be used also in a parallel flow evaporative heat exchanger in which the air travels in the same direction over the coil assembly as the water. The evaporation of the water cools the coil assembly and the internal heat transfer fluid inside the tubes forming the coil assembly.
In accordance with the present invention, the coil assembly comprises an array of closely packed serpentine tubes in which the tubes have two different cross sectional dimensions, preferably when viewed in a horizontal plane. Each tube comprises a plurality of two different types of portions, "segments" and "bights" The "segments" are generally straight tube portions which are connected by the "bights", which are the curved portions, sometimes referred to as return bends, to give the tube its serpentine structure. In the preferred embodiment of the coil assembly of the present invention, the segments of each tube are generally elliptical in cross section and the bights are generally circular in cross section. The generally horizontal diameter of the elliptical segments is smaller than the generally horizontal cross sectional dimension of the generally circular bights. If desired, the bights can have an elliptical cross section, so long as the generally horizontal cross sectional dimension of the segments is less than the generally horizontal cross sectional dimension of the bights. In view of these different cross sectional dimensions, segments of adjacent tubes are always spaced from each other even though the bights of adjacent tubes are in contact with each other. The segments are preferably arranged in generally horizontal rows extending across the flow path of the air and water which flow externally through the coil assembly, whether the air and water are in counterflow or in parallel flow.
The coil assembly of the present invention provides a number of significant advantages. It allows for freer flow of air externally through the coil assembly at lower fan horsepower. It also allows higher spray water flow rates externally over the coil assembly, and thus, higher thermal capacity, without adversely affecting the airflow. It provides for a maximum amount of coil heat transfer surface area within a given coil assembly volume. As a result, the coil assembly provides greater heat transfer capacity. Further, the coil assembly is easy to manufacture and is stronger and more rigid than other designs.
2. Description of The Prior Art
U.S. Pat. Nos. 3,132,190 and 3,265,372 disclose one type of counterflow evaporative heat exchange apparatus in which a coil assembly is mounted in a duct with water sprayed externally downwardly over the coil assembly while air is blown upwardly through the coil assembly. These patents are typical of prior art coil assemblies which will be referred to herein as "tight packed" coil assemblies. In such tight packed coil assemblies, the tubes forming the coils extend in a vertical plane between upper and lower inlet and outlet manifolds in a serpentine manner in which the tubes also extend generally horizontally across the conduit or duct in which the coil assembly is mounted. To maximize the surface area of the tubes being subjected to the external air and water contact, the tubes of the coil are tightly packed together and are in contact with adjacent tubes at the bights and, because the segments and bights have the same cross sectional dimension and shape, they are not spaced apart from each other laterally throughout the entire length of the tube segments. The segments are offset from each other vertically by placing alternate coil circuits at different levels. The open space between two tubes on the same level is equal to the width of the tube in between them. It can be said that a tight packed coil assembly has essentially a 50% open area on each generally horizontal level of segments.
A tight packed coil assembly has the maximum number of tubes that can be built into any given unit width to provide what was thought to be the maximum amount of surface area for a coil assembly for that width. Because of the high number of tubes, the tight packed coil assembly has a relatively low flow of internal fluids flowing within each tube of the coil assembly and a low pressure drop through the interior of the tubes. The airflow pressure drop of the air travelling externally through the coil assembly is relatively high because the tubes are tightly packed together. The external air and water flow through the 50% open area. Spray water flowing down over the coil assembly in a direction opposite the airflow, that is, countercurrent to the airflow, restricts the flow of air to such an extent that the amount of spray water flowing has to be limited as a practical matter to be just enough to wet the coil assembly, but not so much that the airflow rates are adversely affected. Typically, this water flow rate has been limited to values of 11/2 to 3 gallons per minute (gpm) per square foot of plan area. Even for parallel flow equipment, where the external air and water flow in the same direction, the 50% open area is still quite restrictive. Similar to counterflow equipment, water flow rates had to be limited so as not to adversely affect the airflow.
In a effort to improve the heat exchange fluid flow characteristics and heat transfer results, another system was developed and is disclosed in U.S. Pat. No. 4,196,157. The coil assembly used in this system will be referred to herein as a "spaced tube" coil assembly. With a spaced tube coil assembly, the tubes forming the coils have serpentine circuits extending between an upper inlet manifold and a lower outlet manifold while also extending generally horizontally across the duct or conduit of the evaporative heat exchanger in which the coil assembly is mounted. However, rather than packing the tubes so tightly that they contact each other, spacers are used so that laterally adjacent tubes are spaced apart from each other along the entire length of tubes, that is, at both the bights and segments, by a distance comprising a narrow critical range. As in the tight packed coil assemblies, in the spaced tube coil assemblies, the segments are offset from each other vertically by placing alternate coil circuits at different levels. Thus, to provide the efficient heat transfer characteristics disclosed in the patent, the tubes of the spaced tube coil assembly must be spaced apart from each other by an amount such that the space between adjacent tube segments at each horizontal level is greater than the diameter of the tubes but is less than twice the tube diameter. In this type of coil, the open area at any horizontal level could range from slightly greater than 50% to a maximum of 67% and in practice has been approximately 55%.
The spaced tube coil assembly provides certain advantages in counterflow and parallel flow heat exchangers compared to the tight packed coil assembly. The open spaces between the laterally adjacent tubes results in a lower pressure drop requiring a lower fan horsepower to move equal amounts of air externally through the coil assembly than if a tight packed coil assembly were used. It allows the spray water flow to be increased somewhat without an adverse performance penalty on the air fan system.
Despite the claimed improvement in counterflow evaporative heat exchange systems using the spaced tube coil assembly compared to a tight packed coil assembly, there are limitations associated with the spaced tube coil assembly. There is a penalty for the tube spacing in that approximately 20% fewer tubes, and therefore, approximately 20% less surface area, can be built into a given unit width. This results in an approximate 20% higher flow per tube and a corresponding approximate 40% higher pressure drop of fluid flowing internally within the coil assembly. What has been gained by the use of lower fan horsepower and improved airflow externally through the coil is offset by the loss in heat transfer surface area. Nevertheless, in practice, systems employing the spaced tube coil assembly have demonstrated capacities almost the same as the systems using the tight packed coil assembly. The primary advantage of using a spaced tube coil assembly has become a cost savings to the manufacturer due to the fewer number of tubes required.
With the present invention, the advantages of the large amount of surface area of the tubes in a tight packed coil system are combined with the enhanced external air and water flow characteristics of a spaced tube coil assembly to provide a significant increase in heat exchange capacity in an evaporative heat exchanger as compared to equipment of the same size using either a tight packed coil assembly or a spaced tube coil assembly. The present invention results in a real advantage both to the manufacturer of the equipment and the customer by increasing the capacity of a unit of given dimensions.